This invention relates to a method and apparatus for generating and detecting address marks and, more particularly, to a technique by which address marks may be generated and detected by using circuitry that is common to the generating and detecting arrangements, the address mark being adapted for transmission to a remote location or for recording on a magnetic medium, and serving as a reference from which useful information may be detected.
In the field of data communication in general, and magnetic recording in particular, "marks" are helpful in establishing a reference point from which informational data may be detected. While the present invention is readily applicable to data transmission/reception systems of many types, for the purpose of simplification, and to better understand this invention, it is described in the environment of digital magnetic recording.
In digital recording, such as on a magnetic disk, a "mark", commonly referred to as an "address mark", is recorded to indicate the start of a sector in which digital data is recorded. As is conventional, data is recorded in concentric tracks, or "cylinders", on a magnetic disk and each track has one or more sectors recorded therein, although it should be appreciated that a single sector may be comprised of more than one track. Typically, before any information is recorded on the disk, sectors are established so that, when recording is needed, information may be recorded in those sectors. It will be appreciated by those of ordinary skill in the art that such initial formatting of a magnetic disk (or other magnetic recording medium) facilitates the orderly recording and reproduction of useful information.
A typical sector includes an address mark to designate the beginning of that sector, synchronizing information to provide some synchronism between the internal clock of the data recording/reproducing apparatus and the speed at which the recording medium moves (e. g. rotates), an identifying number to identify each respective sector, and data which, typically, is recorded in blocks of 512 bytes. Usually, means are provided to identify the particular sectors in which data are recorded, thereby enab1ing the correct sector to be accessed quickly when that data is to be reproduced.
Of course, the aforementioned address mark should have certain attributes and characteristics that distinguish it from all other data, particularly synchronizing information and informational data, as recorded on the magnetic medium. Various techniques have been proposed heretofore for generating and/or detecting different types of address marks. For example, U.S. Pat. No. 4,345,280 suggests that the address mark be recorded as a high frequency field followed by a low frequency field, this pattern normally not being present in informational data. However, the detection technique described in this patent is relatively complicated to implement in that it calls for a Mealy sequential machine to determine that high frequency pulses of a correct length are followed by low frequency pulses of a correct length. Although the very same read/write heads typically are used to record and reproduce informational data, including the recording and reproduction of the address mark, this patent contemplates the use of different devices for address mark recording and for address mark detection. Hence, not only is the technique described in this patent rather difficult to implement, it also is relatively expensive.
The utility of address marks is found not only in digital data recording on magnetic disks but also in the field of video recording on magnetic tape. In U.S. Pat. No. 3,739,086, a "frame boundary signal" formed of a sequence of alternating "1"s and "0"s interposed between beginning and ending pairs of "1"s is recorded. Presumably, the pattern formed of this particular sequence normally is not found in useful data. Here, the frame boundary signal is detected by sensing a portion of the alternating "1"/"0" sequence. However, it is believed that, in view of this particular pattern, serious constraints are placed upon the data pattern that may be recorded in addition to the frame boundary signal. Furthermore, since only a portion of the alternating bit sequence need be detected, it is possible that spurious errors that are introduced into the useful data for a brief period of time may be interpreted falsely as a frame boundary signal.
To provide high density magnetic recording of digital data, it is proposed in U.S. Pat. No. 4,367,497 to record a preamble consisting of fifteen successive "1"s, followed by useful data which, in turn, is followed by a mirror-image postamble. However, it appears that this particular preamble/postamble pattern was adopted specifically for the NRZI code but is not necessarily satisfactory when utilizing the conventional codes (such as the 2,7 code) that have been adopted in conventional digital data handling apparatus now commercially available.
Various additional techniques and patterns have been proposed for generating unique address marks. Nevertheless, various difficulties and disadvantages are associated with those techniques and patterns. In particular, there has been a need for generating unique address marks that are easily distinguishable from informational data, particularly from data that is recorded in, for example, 2,7 or MFM code which normally is used for recording digital data on magnetic disks. Moreover, it is important that such address marks be generated and detected by apparatus that is reliable but is relatively inexpensive. Preferably, circuitry that is used for address mark generation also should be used for address mark detection. These desirable features are achieved by the present invention.